officer for the city heading up chicago's ambitious mission to cool the city down. >> we're in the midst of, right now, a $7 billion multi-year program on building a new chicago. so we're redoing and renovating infrastructure throughout the city. >> sreenivasan: they're changing everything from head to toe-- or, in a city's case, from roof to street. one of chicago's most beautiful gardens is one very few people get to see, a 23,000-square-foot green roof that sits on top of city hall. the difference a green roof makes is measurable. that side of city hall's roof used to be traditional black, like most roofs. on a hot day, when it was 90 degrees out, they came out and measured the surface temperature. it was 169 degrees, versus this side of city hall, where they have a green roof, and the surface temperature was 90 degrees. michael berkshire administers green roof projects for the city. >> rooftops are one of the last kind of frontiers that you can really look at, and it's a significant amount of space. >> sreenivasan: berkshire says the plants on green roofs soak up heat, and keep both